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Introduction

This chapter introduces the motivation for this consensus report. It begins with an overview of fires at the wildland-urban interface (WUI) and why they deserve attention. It discusses prior work, which this report builds upon, that helps frame the issue. It presents the statement of task for the National Academies of Sciences, Engineering, and Medicine (the National Academies) committee responsible for the findings and recommendations in this report and the committee’s approach to this task, including its consideration of equity issues surrounding data availability and disparities in the impact of these fires. The chapter concludes with a description of the organization of the rest of the report.

WILDFIRES AT THE WUI

Wildfire activity in the United States is increasing as more frequent extreme weather events, including heat waves, droughts, and lightning activity, cause wildfires (Ahrens, 2013; Schwartz and Penney, 2020). Combined extreme heat and drought conditions have occurred more frequently across North America over the last century (Alizadeh et al., 2020), and fire seasons have lengthened by a global average of 18 percent from 1979 to 2013.¹ Growth in wildland fire size and intensity has also been driven by historical land management practices that emphasized fire exclusion and allowed the buildup of potential wildland fire fuels. Simultaneously, the WUI has continued to expand further into formerly wilderness areas (Radeloff et al., 2018). Since 1990, 41 percent of new housing units in the United States have been built in the WUI (Radeloff et al., 2018). Climate change is inextricably linked to these issues, as it contributes to extreme weather conditions that increase fire activity and is affected by land use changes like the expansion of the WUI (USGCRP, 2018). The combination of these factors has led to an increase in WUI fires.

The chemistry and ultimate health impacts of WUI fires are still poorly understood. WUI fires lead to higher human exposures than remote wildland fires because of their proximity to communities. They also have unique chemistry due to the combination of natural and human-made fuels that are burned, which may lead to the formation or release of toxic emissions not found in purely wildland fires.

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¹ Fire season is defined by the US Forest Service as “1) Period(s) of the year during which wildland fires are likely to occur, spread, and affect resource values sufficient to warrant organized fire management activities. 2) A legally enacted time during which burning activities are regulated by state or local authority” (see https://www.fs.fed.us/nwacfire/home/terminology.html).

Economic impacts of WUI fires are also significant. A 2017 report by the National Institute of Standards and Technology (NIST) estimated the economic impacts associated with wildfires in the United States, including WUI fires, as between US$71 billion and US$347 billion per year. These economic impacts include both costs, such as preventive measures and disaster response, and losses, such as injuries, mental health impacts, disease or exacerbation of existing health conditions, and deaths. Economic valuation of loss of life accounted for US$28 billion to US$202 billion of the total economic impacts (Thomas et al., 2017). Chapter 2 expands on this discussion of the definition, causes, and impacts of WUI fires.

The expanding problem of WUI fires has led to research targeted at understanding fire dynamics and fire progression at the WUI. Post-incident analyses of operational responses, disaster management, and risk communication associated with WUI fires have also become more detailed as more real-time incident data are collected (Gaudet et al., 2020). The highly interdisciplinary character of this problem has led to the involvement of multiple agencies at the federal level for scientific research, emergency preparedness, and disaster response. The Wildland-Urban Interface Fire Group at NIST conducts research to understand and predict the spread of WUI fires, including the investigation and reconstruction of key case studies like California’s Camp Fire in 2018 (Maranghides et al., 2021). The Centers for Disease Control and Prevention (CDC) runs multiple programs dedicated to wildfire disaster preparedness and response through their National Center for Environmental Health and Natural Disasters and Severe Weather division. The National Institute of Environmental Health Sciences (NIEHS) operates a Worker Training Program to support the health and safety of people involved in wildfire response operations. CDC and NIEHS recognize that those individuals whose occupations bring them in proximity to wildfires and their aftermath are increasingly being exposed to WUI-type fires.

To enhance their efforts, CDC, NIEHS, and NIST asked the National Academies to evaluate chemistry information that would improve the mitigation of acute and long-term health effects of residential burning during wildfires and to recommend chemistry research that could help to inform decision-makers charged with mitigating wildfire impacts on the general public. As a result of the request, the National Academies convened the Committee on the Chemistry of Urban Wildfires, which prepared this report.

THE COMMITTEE AND ITS TASK

The committee convened in response to the sponsors’ request included experts in atmospheric chemistry, combustion chemistry, environmental chemistry, toxicology, and exposure science. Appendix B provides biographical information on the committee. The committee was asked to review the relevant state of the science and recommend chemistry research that could fill critical data gaps and help inform decision-makers charged with mitigating wildfire impacts on the general public. The Statement of Task is provided in Box 1-1.

THE COMMITTEE’S APPROACH TO ITS TASK

To accomplish its task, the committee convened a public workshop to gather information relevant to its task and held seven committee meetings to deliberate and reach its conclusions and recommendations. In addition to the workshop, committee members used their subject-matter expertise to independently compile available literature specific to WUI fires and, when in their judgment it was relevant, non-WUI wildfires. Early in its work, the committee identified the broadly interdisciplinary nature of its task as a barrier to applying consistency to the reporting of available evidence. For example, the types of studies available about combustion conditions and emission factors include laboratory-, bench-, and field-scale studies and often report dimensionless ratios, while studies examining health effects fall into different categories (e.g., epidemiological, observational, in vivo). The committee recognized that this problem exists in many interdisciplinary fields of research and is not unique to WUI fire chemistry; creating a new system for integration of the best available scientific information in each discipline was deemed beyond the study’s scope. Therefore, the committee proceeded to use the highest-quality evidence in each scientific discipline while recognizing the eventual need to integrate information across multiple scientific disciplines. The report prioritizes information available in peer-reviewed sources.

The committee considered various issues and made several decisions in interpreting its charge. First, it considered the sequence of chemical processes that occur, from the initial burning of fuel sources in WUI fires to the chemical products generated by the fires. In considering fuel sources for fires at the WUI, the committee recognized that burned areas could include residential, commercial, and industrial sources but focused its attention on materials present in residences, because residences constitute a large fraction of structures burned at the WUI. The committee also recognized that combustion products depend on the intensity of the fire and the nature of heat transfer to fuels of both natural and human-made origin. The committee understood that fires at the WUI can be more intense than urban residential fires and that WUI fires often burn structures from the outside in, rather than the typical pattern of residential fires burning from the inside out. The committee recognized potential synergies in the combustion chemistry associated with materials involved in WUI fires: the chemical processes associated with the combustion of wildland fuels and fuels associated with the built environment will interact in WUI fires in potentially important ways.

Second, the committee considered human exposures during the active phase of the fire and post-fire activities. The populations it considered included the local communities in the burned region, first responders, communities that are exposed to regional plumes, and people who are involved in clean-up activities. The wide distribution of exposed communities made it important for the committee to consider chemical transformations of the initial combustion products over local and regional scales, and how those transformations are influenced by local and regional environmental conditions. Although a primary focus was on inhalation exposures, the committee also considered ingestion and dermal exposure routes that could be important in the exposed populations. The committee recognized the importance of considering affected communities through the lens of equity and environmental justice. Because the Statement of Task focuses on human exposure and health impacts, the committee decided that discussions of ecological impacts and long-term environmental fate were out of scope for this report.

Third, the committee recognized that WUI fires have the potential to generate a wide array of chemicals and that many chemical exposure pathways are relevant. To address its charge of identifying critical information gaps, the committee qualitatively considered categories of potential health effects associated with exposures but did not attempt to quantify effects where such analyses did not already exist in the literature. Exposures can be acute or chronic, depending on their magnitude and duration, and health effects can be acute or chronic, or have a delayed onset. The committee identified a number of potential effects on populations in the near-fire area, regionally, and on continental scales.

Finally, although the work of the committee can inform mitigation measures such as reducing the use of materials in structures at the WUI that might lead to combustion products that have adverse health effects, the committee did not directly evaluate mitigation policies or measures. Because the work of the committee assesses what is known and identifies critical information gaps associated with exposures to chemical releases from WUI fires, this report can help to inform communications with exposed populations during fire events.

THE COMMITTEE’S APPROACH TO EQUITY

The committee recognized that its discussions, findings, and recommendations must involve intentional considerations of equity and, during the course of the study, aimed to identify inequities and structural barriers and describe science-based opportunities to promote greater access and improved outcomes for vulnerable and marginalized groups. The committee acknowledges that health equity is a much larger issue that is a factor in types of exposures other than WUI fire exposure; however, it believes it is important to address in this report as it relates to the committee’s task.

The disproportionate vulnerability of certain groups to effects from WUI fires can be seen through a few simple examples:

1. Smoke impacts community air quality at a regional and even continental scale, far outside the evacuation zone. Individuals’ inhalation exposure to smoke is dependent in part on how airtight their dwelling is (how recently it was built; whether there is a heating, ventilating, and air-conditioning system) and whether they have the financial means to reduce indoor concentrations of smoke or to relocate to an area with better air quality.
2. Exposure to pollutants from WUI fires is increased for people who work outdoors. These jobs tend to be more physically demanding, increasing respiratory exchange with polluted air and therefore inhalation exposure.
3. Children, pregnant people, older adults, and people with preexisting conditions all have heightened susceptibility to health effects from exposure to toxicants from WUI fires, through inhalation or other routes of exposure.

The committee discussed inequity throughout the report; however, the majority of the report’s related findings and recommendations can be found in Chapter 6, “Human Exposures, Health Impacts, and Mitigation.”

PRIOR RELATED WORK BY THE NATIONAL ACADEMIES

This study focused specifically on fires at the WUI; however, the committee recognizes that a great deal can be learned from studying wildland fire research. The National Academies has held three workshops in recent years to gather experts together and share knowledge related to specific aspects of wildfire issues in the United States. A 2017 workshop and proceedings, titled A Century of Wildland Fire Research: Contributions to Long-Term Approaches for Wildland Fire Management, focused on fuel management and preventative measures that wildland fire managers, policy makers, and communities can implement to decrease wildfire incidence. A 2019 workshop and 2020 proceedings, titled Implications of the California Wildfires for Health, Communities, and Preparedness, focused on similar issues specific to California and expanded to include human health impacts of wildfires in the state. Most recently, a 2020 workshop and 2022 proceedings on Wildland Fires: Toward Improved Understanding and Forecasting of Air Quality Impacts focused on atmospheric chemistry and exposure science but specifically excluded the WUI. These three prior efforts, while touching on topics related to chemistry and the WUI, did not focus on WUI fires or their chemistry. This report builds on these workshops and focuses on the specific problem of the chemistry of WUI fires.

ORGANIZATION OF THIS REPORT

This report is organized into eight chapters. Chapter 2 describes the history and characteristics of the WUI and fires at the WUI, and provides working definitions for key terms that the committee uses throughout the report. It concludes with five context-setting examples of recent, major WUI fires.

Chapter 3 covers the materials, combustion, and emissions of fires at the WUI, emphasizing the differences between urban materials (often with higher concentrations of toxicants) and the vegetative biomass of wildlands. It summarizes what is known about the materials in the WUI environment, the combustion conditions, the dominant reaction pathways, and the resulting emissions from WUI fires. It highlights the complexity of the many

intersecting factors that influence emissions: the material types and loading, and the arrangement of materials in the landscape; the many reaction pathways and the effects of conditions like temperature; and the combustion chemistry unique to WUI fires.

WUI fires can affect populations at great distances away from the fire. Chapter 4 discusses the atmospheric transport and chemical transformations of WUI fire emissions. The chapter explores the atmospheric chemistry and physical changes (such as dilution) that lead to dramatic changes in the pollutant composition downwind of fires, although little data exist on these processes for WUI fires. A section covers current practices in modeling fire plumes as they move away from the fire. Chapter 4 emphasizes the atmospheric chemistry that informs inhalation exposures and the resulting health effects associated with WUI fires.

While emissions from WUI fires can stay in the air, they also end up in the water and soil. Chapter 5 covers water and soil contamination, both near the fire and downwind. It discusses immediate impacts in the fire region and effects on community water systems, groundwater, and soil, as well as the deposition of contaminants downwind of a fire. It includes post-fire contamination associated with firefighting activities. Due to the lack of information on WUI fires, in some cases, information originally collected for wildland fires is extrapolated to WUI fires.

Chapter 6 explores pollutants of concern for human exposure, environmental justice, and vulnerable populations, as well as ways to reduce exposure and mitigate the impacts of WUI fires. It addresses distance scales from the immediate fire zone, where people including firefighters experience direct exposure to heat and fire emissions, to the regional and continental levels, where WUI fire smoke can have an extended impact. It explores routes of exposure and the acute and chronic health impacts for all near- and far-field populations, including firefighters. It discusses specific interventions for firefighters and affected communities. The chapter includes information from both wildland fires and, where available, WUI fires.

Chapter 7 describes the methods for collecting data on WUI fires and their emissions, focusing on the overall challenges in data collection and on the strengths and limitations of currently available techniques. Current methods include measuring the fire’s emissions at near and far locations and measuring exposure to humans via sampling the nearby environment, personal exposure measurements, and biomonitoring. The chapter also describes the analytical instruments and modeling techniques used.

Chapter 8 summarizes the report and the information needs it identifies. It stresses the wide variety of types of information needed and the importance of multidisciplinary research teams. It presents the information needs as a research agenda. This agenda groups complementary tasks together and organizes priorities into four primary areas: (1) fuels and emissions; (2) chemistry, transport, and transformations; (3) exposure and health; and (4) measurement science and analytics.

The report concludes with five appendices. Appendix A provides a glossary of key terms used in this report. Appendix B presents biographical information about the committee. Appendix C contains a table summarizing the available data about the example fires described in Chapter 2 as a list of references. Appendix D contains the agenda for the public session held by the committee for information gathering purposes. Appendix E lays out the assumptions and engineering calculations that the committee used to create a table of combustible mass and energy content data in Chapter 3.

REFERENCES